1. Field of the Invention
This invention relates to the recovery of ethanol from dilute aqueous solutions by liquid-liquid extraction employing a variety of liquid extraction media, and is especially applicable to recovering and concentrating ethanol present in the dilute aqueous solutions obtained by fermentation.
2. Description of the Prior Art
With the ever-increasing depletion of economically recoverable petroleum reserves, the production of ethanol from vegetative sources as a partial or complete replacement for conventional fossil-based liquid fuels becomes more attractive. In some areas, the economic and technical feasibility of using a 90% unleaded gasoline-10% anhydrous ethanol blend ("gasohol") has shown encouraging results. According to a recent study, gasohol powered automobiles have averaged a 5% reduction in fuel compared to unleaded gasoline powered vehicles and have emitted one-third less carbon monoxide than the latter. In addition to offering promise as a practical and efficient fuel, biomass-derived ethanol in large quantities and at a competitive price has the potential in some areas for replacing certain petroleum-based chemical feedstocks. Thus, for example, ethanol can be catalytically dehydrated to ethylene, one of the most important of all chemical raw materials both in terms of quantity consumed and versatility in product synthesis.
The various operations in processes for obtaining ethanol from such recurring sources as cellulose, cane sugar, amylaceous grains and tubers, e.g., the separation of starch granules from non-carbohydrate plant matter and other extraneous substances, the chemical and/or enzymatic hydrolysis of starch to fermentable sugar (liquefaction and saccharification), the fermentation of sugar to a dilute solution of ethanol ("beer") and the separation and concentration of the ethanol by distillation, have been modified in numerous ways to achieve improvements in product yield, production rates and so forth (see, for example, U.S. Pat. No. 3,236,740 and the booklet "Industrial Alcohol by Continuous Fermentation and Vacuum Distillation With Low Energy Consumption," of Chemapec, Inc., Woodbury, N.Y.). For ethanol to realize its vast potential as a partial or total substitute for petroleum fuels or as a substitute chemical feedstock, it is necessary that the manufacturing process be as efficient in the use of energy and raw materials as possible so as to maximize the energy return for the amount of ethanol and enhance the standing of the ethanol as an economically viable replacement for petroleum based raw materials. To date, however, relatively little concern has been given to the energy and raw material requirements for manufacturing ethanol from biomass and consequently, little effort has been made to minimize the thermal expenditure and waste of raw materials incurred in carrying out any of the aforesaid discrete operations involved in the manufacture of ethanol from vegetative sources.
Recovery of fermentation ethanol by distillation accounts for a large amount of the overal energy requirements for conversion of biomass to concentrated ethanol. Roddy, "Distribution of Ethanol-Water Mixtures to Organic Liquids," Ind. Eng. Process Des. Dev., 1981, 20, 104-108, proposes the use of organic solvent extraction followed by gas stripping of ethanol from the organic phase as a substitute for distillation in alcohol separation and concentration. According to this publication, the hydrocarbons as a class are poor extractants for ethanol but tend to give the highest separation factors because of their even poorer solvent properties for water. A variety of ethanol extractants were evaluated including cyclohexane, benzene, toluene, xylene, ethylbenzene, chloroform, 1-octanol, 2-ethyl-1-butanol, n-butyl acetate and tri-n-butyl phosphate, and their distribution coefficients K (i.e., the value obtained by dividing the concentration of ethanol in the organic layer by the concentration of ethanol in the aqueous layer) were measured. Roddy discloses that the highest distribution coefficient (designated by the author as D.sub.EtoH) was 6.9.times.10.sup.-1 (measured at 25.degree. C.) which was obtained for 2-ethyl-1-butanol. The only nitrogen-containing extractants evaluated by Roddy, Amberlite XLA3, a primary amine from Rohm and Haas, and Adogens 364 and 464, tertiary and quaternary amines from Ashland Chem., had much lower distribution coefficients, measuring 4.4.times.10.sup.-3, 1.7.times.10.sup.-2 and 4.8.times.10.sup.-1 respectively.